1. Field of the Invention
This invention relates to a device for detecting the deterioration of a catalyst in an internal combustion engine wherein air-fuel ratio sensors (oxygen sensors) are provided on the upstream side and the downstream side of a catalytic converter installed for purifying the exhaust gas in an internal combustion engine, and for alarming a driver.
2. Discussion of Background
Conventionally, in the fuel injection control of an internal combustion engine, a basic injection quantity of a fuel injection valve is calculated in accordance with an intake quantity and a revolution at speed of the engine. The basic injection quantity is corrected in accordance with an air-fuel ratio correction calculated based on a detected signal of an oxygen sensor for detecting an oxygen concentration in the exhaust gas. An actually supplied fuel quantity is controlled in accordance with the corrected injection quantity. This control is repeatedly performed, which finally converges the air-fuel ratio of the engine in a predetermined range.
In the fuel injection control, the air-fuel ratio is controlled within a very narrow range adjacent to a theoretical air-fuel ratio by a feedback control with respect to the air-fuel ratio. Furthermore, the purification capability of a three-way catalytic converter provided at the exhaust system, that is, a capability for converting poisonous components in the exhaust gas; CO, NO.sub.x and HC to harmless ones by oxidation and reduction reactions, can be highly maintained.
In the above air-fuel ratio control, the heating or poisoning of the exhaust gas lowers a response speed of the oxygen sensor, lowers an output voltage thereof, and elevates an activation temperature thereof. With respect to the catalyst, the lowering of its purification efficiency and elevation of its activation temperature thereof are caused. A measure against the lowering of the performances, is described, for instance, in Japanese Unexamined Patent Publication 197737/1986, as follows.
(a) Oxygen sensors are provided respectively upstream and downstream from a catalyst and an activation determining means is provided. The air-fuel control is performed by using the two oxygen sensors or either activated one of the oxygen sensors.
(b) With deterioration of a response characteristic of an oxygen sensor, a control period of a feedback control of the air-fuel ratio is accelerated, thereby preventing deterioration in a response characteristic of the total system.
However, the conventional device responds to deterioration of the oxygen sensor, and not to deterioration or destruction of the catalyst per se. Accordingly, since a driver can not detect a failure, he runs a vehicle emitting poisonous substances.